This Background section is provided for informational purposes only, and should not be considered as an admission that any of the material contained in this section qualifies as prior art to the present application.
There is a need for conveying data between two separate underwater entities in applications including defense, oceanography, hydrocarbon development, etc. Conventional methods for conveying data between underwater entities employ either a tethered link using copper or fiber optics, or rely on acoustic transmission. According to the former approach, the underwater entities must be repositioned or replaced in situ, while the latter approach has a very low data rate (1 to 20 kilobits per second is typical) that is currently possible using acoustic transmission. An approach that uses light propagating freely in the ocean environment would provide much higher data rates and the possibility of conveniently exchanging data between arbitrary pairs of transmitting and receiving devices (transceivers).
Some attempts to implement data transmission between underwater entities using optical means have been frustrated by a lack of suitable light sources. The propagation of light through water is limited by the fundamental absorption properties of pure water, scattering of particulates such as plankton and inorganic particulates, and absorption by chlorophyll-containing phytoplankton and other organic materials. The components combine, in various combinations, to favor strongly the transmission of light in the blue-green region of the optical spectrum, approximately from 400 to 600 nm. The optical effect of the various combinations of the components admixed in water can be summarized as water types and range from the very purest natural waters, which favor deep blue propagation (nominally 450 nm), to waters which favor blue-green (nominally 490 nm) and green (nominally 530 nm) propagation. The minimum optical attenuation coefficients at the optimal wavelengths vary from about 0.02 m-1 for the very clearest natural waters, to more than 2 m-1 in the most turbid coastal or harbor waters.
Previous light sources in the blue-green wavelength range have included been bulky, inefficient, expensive and employed external modulators.